GCPSigner: Add exception for unsupported keys

This seems correct-er and makes pylint happier.

Signed-off-by: Jussi Kukkonen <[email protected]>

securesystemslib/signer/_gcp_signer.py

@@ -173,6 +173,10 @@ def _get_hash_algorithm(public_key: Key) -> str:
             # nistp256 uses sha-256, nistp384 uses sha-384
             bits = public_key.scheme.split("-nistp")[-1]
             algo = f"sha{bits}"
+        else:
+            raise exceptions.UnsupportedAlgorithmError(
+                f"Unsupported key type {public_key.keytype} in key {public_key.keyid}"
+            )
 
         # trigger UnsupportedAlgorithm if appropriate
         _ = sslib_hash.digest(algo)